Many circuits, networks, electrical devices and data handling systems are operated in configurations and environments where external factors can impair their performance, cause failure or even result in permanent damage. Among the most common of these factors are over-voltage and over-current. Protection against these factors is important and has been addressed in the prior art in a number of ways, depending on the specific electronics and their application.
Fuses that employ thermal or magnetic elements are one common protection measure. In other cases, protection circuits are available. Some examples are described in U.S. Pat. Nos. 5,130,262; 5,625,519; 6,157,529; 6,828,842 and 6,898,060. Protection circuits are further specialized depending on conditions and application. For example, in the case of protecting batteries or rechargeable elements from overcharging and over-discharging one can refer to circuit solutions described in U.S. Pat. Nos. 5,789,900; 6,313,610; 6,331,763; 6,518,731; 6,914,416; 6,948,078; 6,958,591 and U.S. Published Application 2001/00210192. Still other protection circuits, e.g., ones associated with power converters for IC circuits and devices that need to control device parameters and electric parameters simultaneously also use these elements. Examples can be found in U.S. Pat. Nos. 5,929,665; 6,768,623; 6,855,988; 6,861,828.
When providing protection for very sensitive circuits, such as those encountered in telecommunications the performance parameters of the fuses and protection circuits are frequently insufficient. A prior art solution embodied by transient blocking units (TBUs) that satisfy a number of the constraints is taught in international publications PCT/AU94/00358; PCT/AU04/00117; PCT/AU03/00175; PCT/AU03/00848 as well as U.S. Pat. Nos. 4,533,970; 5,742,463 and related literature cited in these references.
Unfortunately, when the trigger currents in TBUs are low their performance is limited. Specifically, the lower limit of TBU resistance is in the 8-10Ω range at trigger currents of 180 mA or less. A lower resistance is desirable in many applications mainly because of lower voltage drop, reduced power loss and higher system efficiency. Lower resistances are possible if either the trip current is increased or if the pinch-off voltage in traditional TBUs is reduced.
Unfortunately, this is not an option in many high-sensitivity applications.